The invention relates to a device for parameter-dependent control of a reaction force at an operating element, e.g. steering wheel, of a hydraulic power steering system and, more particularly, to a device having an electric motor which can be coupled in a driving manner to the operating element, which electric motor is not self-locking.
A hydraulic power steering system of the type described above is known from German Patent document DE 39 08 218 A1. In this known reference, an electric motor serving to generate a restoring torque can be coupled to the steering wheel.
Today's passenger cars are primarily outfitted with hydraulic power steering systems in order to keep the driver applied steering forces on the operating element, i.e., the steering wheel, sufficiently low. This is done, in particular when driving very slowly, for example during maneuvering of the vehicle. Power steering systems of this type have, in the mechanical drive transmission between the steering wheel and the steered wheels of the vehicle, two transmission elements. The two transmission elements are movable relative to one another to a limited extent counter to a spring force. Correspondingly, the two transmission elements carry out greater or lesser relative movements in dependence on the forces transmitted between the steering wheel and the steered wheels. These relative movements control a servo valve which is arranged between a hydraulic pressure source and a relatively pressureless hydraulic reservoir and have essentially two parallel throttle routes. Each throttle route has an input throttle and an output throttle. The input and output throttles of each throttle route are controlled in opposite directions. The two throttle routes are incidentally also controlled in opposite directions.
Additionally, it is a disadvantage that the hydraulic reaction members bring about a specific additional friction which can lead to hysteresis effects and to a poorer return of the steering.
Alternatively to hydraulic power steering systems, electromechanical power steering systems are also known, in which the servo positioning force is generated using an electric motor. However, in view of the servo positioning forces which are generated, in these known systems the vehicle's electrical network must be designed to have increased capability. Additionally, it is a disadvantage with electromechanical systems that accurate controlling of the servo positioning forces, or providing desired magnitudes for the forces, causes control difficulties even in very rapid steering maneuvers. Among other things, this is based on the fact that, in view of the small dimensions of the electric servomotor, high-speed electric motors with a comparatively low torque are generally used. The high-speed electric motors can be connected to the steering via separable drive paths. It is thus possible to allow the electric servomotor to run constantly at a higher speed and to couple it non-positively to the steering system if required.
There is therefore needed a new design for the reaction control of a power steering system.
This need is met according to the present invention in that, in the device of the type described above, the electric motor can be controlled parameter dependently in a manner analogous to a variable which changes in a manner analogous to the actuating force applied at the operating element.
The present invention is based on the general concept of departing from hydraulic control of the reaction forces in dependence on parameters to be determined in a hydraulic power steering system and, instead, providing an additional electric motor which can be controlled in dependence on the parameters.
The advantage achieved according to the present invention is that customary hydraulic power steering systems can be used without parameter-dependent hydraulic control of reaction forces. Servo systems of this type are mass produced and are thus cost-effective. In particular, in a large-series production, both standard vehicles without parameter-dependent control of the reaction forces of the steering system and vehicles with special fittings, in particular with a power steering system with parameter dependently controlled reaction forces, can be equipped with the same hydraulic servo systems. In the case of retrofittings, only the electric motor with its control needs to be additionally provided. In total, significant cost advantages thus result because all the vehicles, in principle, have systems of the same type and only additional systems have to be assembled in the event of special or retrofittings.
According to a preferred embodiment of the present invention, the power steering system has a servo valve (which is known in principle) which is arranged between a hydraulic pressure source and a relatively pressureless hydraulic reservoir. The servo controls a hydraulic servomotor and can be controlled by relative movements between parts of a mechanical drive transmission which couples the operating element or the steering wheel to control members, in particular the steered wheels of the vehicle. In this case, the electric motor is coupled in a driving manner to the operating-element-side part of the parts which are movable relative to one another.
In this manner, it is guaranteed that the forces generated by the electric motor cannot have any direct effect on the servo positioning forces generated by the hydraulic servomotor.
For power steering systems of a motor vehicle, it is generally preferred to design the hydraulic servo system such that it is only capable of generating lower servo positioning forces which are desired at relatively high vehicle speeds. The electric motor is then used to generate an additional assisting force to be used at slower speeds or in particular situations.
In principle, however, it is also possible to design the hydraulic servo system to have such a capability that it is capable of generating the highest servo forces desired during operation, and that, depending on the Parameters, the electric motor generates an additional resistance which can be felt at the operating element.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.